This invention relates to pile drivers arranged to be operated by a compressible fluid and is concerned with reducing the noise of operation of such machines.
A well-recognised disadvantage of conventional pile drivers is that they are very noisy in operation. This is partly because they rely on using a large mass as a hammer or ram to strike an anvil on the pile to drive the pile into the ground, and in the case of drivers operated by compressible fluid such as compressed air or steam partly because of the release and exhaust of pressure fluid at the required stages of a reciprocating cycle. Various attempts have been made to lessen the noise of operation, e.g. by constructing a sound-insulating enclosure within which the hammer or ram moves but this is extremely cumbersome.
It is known to place non-metallic shock-absorbing blocks or dollies between the striking parts to avoid metal-to-metal contact but this does not substantially reduce noise and does not have any effect at all on the noise produced by release of pressure fluid.
The admission of gas under pressure has also been proposed for producing a fluid cushion that prevents direct impact. In one arrangement (U.S. Pat. No. 3,714,789) compressible fluid is injected under the falling mass shortly before it reaches the bottom of its reciprocating stroke to produce a fluid cushion decelerating the mass and then accelerating its initial return upwards, after which the pressurised cushioning fluid is released. This proposed arrangement is very complex and therefore expensive to manufacture. In another arrangement (UK Pat. No. 1 396 575) a ram cylinder is alternately connected to a pressure source and to exhaust in order to produce a series of pressure pulses underneath a weighted ram piston to generate a periodic driving force, the pressure being so regulated that the ram piston is kept out of contact from a dolly or impact-absorbing pad on the pile. This result can only be achieved when the ground resistance to the pile is low because the peak cushioning pressures are limited by the delivery pressure of the pump supplying the ram, there being a return flow through the pressure delivery line when the cushion pressure exceeds the normal line pressure.